As a background art of the present technical field, for example, Japanese Patent Application Laid-Open Publication No. 2010-40698 (Patent Document 1) is cited. This Patent Document 1 describes as follows. “The SBD is provided with: an n-type GaN layer formed on a GaN substrate; a Schottky electrode formed thereon; and a back-surface electrode. On the n-type GaN layer, a guard ring having a regular hexagonal ring shape and surrounding the Schottky electrode is provided. The guard ring is formed of a p-type GaN. The Schottky electrode is in contact with an inner side surface and an upper surface of the guard ring in an ohmic contact. A side surface of the guard ring is an “m” plane. The side surface of the guard ring is patterned by plasma etching, and then, is subjected to anisotropic wet etching. By this structure, a leakage current is decreased, and a breakdown voltage is increased” (see the abstract).